[Support] add predicate_string

[agileseph]

This PR dedicated to adding predicate_string Query specifier. So it allows using matchingPredicate in a natural way as XCTest provides it with XCUIElementQuery class. The predicate will be evaluated against objects of type id.
The basic negative use cases include:

1. If an attribute used incorrectly in predicate string, the app is crashed. So validator should be added a bit later.
2. if the syntax is incorrect for elementType attribute, the user receives expected error with example.
3. if an element type specified incorrect, the user receives expected error about incorrect type.